Various types of electrolytes are being used for contemporary electrochemical devices, for example, lithium secondary batteries, electrolytic condensers, electric double-layer capacitors and electrochromic display devices, and for dye-sensitized solar cells of which various studies are being undertaken for future commercialization, and the importance of electrolytes is increasing day by day.
In particular, lithium secondary batteries have high energy density and long life cycle, and thus are at the center of attention. Generally, a lithium secondary battery comprises an anode made from carbon materials or lithium metal alloys, a cathode made from lithium metal oxides, and an electrolyte made by dissolving a lithium salt in an organic solvent. The structural stability and capacity of lithium metal oxides are determined by intercalation and disintercalation reactions of lithium ions. As the charge potential rises, lithium metal oxides increase in capacity but become structurally unstable. This unstable structure of an electrode provokes the generation of oxygen, which may cause overheating in a battery or a reaction with an electrolyte, possibly resulting in the battery exploding.
Currently, organic solvents used widely for an electrolyte of lithium secondary batteries include ethylene carbonate, propylene carbonate, dimethoxy ethane, gamma-butyrolactone (GBL), N,N-dimethyl formamide, tetrahydrofurane, acetonitrile, and the like. Because these organic solvents generally have high volatility and high ignitability, lithium secondary batteries using the same may have problems in stability, especially at high temperature.
In order to solve this problem, suggestions have been made to use imidazolium-based or ammonium-based ionic liquids as an electrolyte of lithium secondary batteries. However, the ionic liquids may be reduced in an anode at a higher voltage than lithium ions, or imidazolium or ammonium cations may be inserted into an anode together with lithium ions, which may rather deteriorate the performance of the battery.
Meanwhile, Korean Patent Registration No. 10-751203 and Korean Laid-open Patent Publication No. 10-2007-85575 disclose, as an electrolyte, a eutectic mixture of a lithium salt and an amide compound such as acetamide, urea, methylurea, caprolactam, valerolactam, trifluoroacetamide, carbamate, formamide, and the like, which is represented by a certain chemical formula. The eutectic mixture exhibits high thermal and chemical stability as well as a relatively wide electrochemical window, and thus, solves the problems such as evaporation or ignition of an electrolyte caused by the use of conventional organic solvents.
As described above, the development of various electrolytes is fast growing. In particular, there is an increasing demand for an electrolyte containing a compound having higher stability at high temperature and a low minimum limit of an electrochemical window, suitable for applications to an electrochemical device of which various electrochemical characteristics are required.